The present invention relates to a ratio control system for a continuously variable transmission drivingly connected to an engine of a motor vehicle having a manually operable accelerator. The engine has a throttle operatively connected to the accelerator such that the throttle opens in degrees as the accelerator is depressed.
A continuously variable transmission including a fluid coupling and a pulley unit with a V-belt is known from a Japanese patent application No. 59-226706 laid open for public inspection under JP 61-105351 or a copending U.S. patent application No. 922,400 filed Oct. 23, 1986 (now U.S. Pat. No. 4,735,113 issued on Apr. 5, 1988 to Yamamuro et al.) and commonly assigned herewith. In this known continuously variable transmission, the pulley unit is hydraulically actuated by a shift control valve that is operatively linked via a shift operation mechanism to a shift actuator. The shift actuator is controlled by a microcomputer based control unit which receives various input information signals including an engine revolution speed signal, a vehicle speed signal and a throttle opening degree signal. Among them the vehicle speed signal and the throttle opening degree signal are used to determine a target engine revolution speed and produces an output signal based on a result of comparison between the actual engine revolution speed indicated by the engine revolution speed signal and the target engine revolution speed signal. The actuator under the command of the output signal actuates the shift control valve to thereby regulate the fluid supply to and fluid discharge from a driver pulley of the pulley unit in order to adjust a speed ratio in such a direction as to reduce a difference between the actual engine speed and the target engine speed toward zero.
The control characteristic of this known ratio control system is illustrated by a linear throttle opening degree TVO versus a reduction ratio i characteristic as shown in FIG. 14. According to this characteristic, cyclic depression of an accelerator pedal and release of the acelerator pedal causes a substantial variation in an output shaft torque of the transmission. Such cyclic depression and release of the accelerator pedal is required when a vehicle operator wishes to maintain a certain vehicle speed against varying running resistance. This torque variation is uncomfortable to passengers and the operator of the vehicle, and in effect decreases ride comfort.
Describing in detail, as shown in FIG. 14, a small change (dTVO) in the throttle opening degree TVO causes a change (di) in the reduction ratio i. Referring also to FIG. 15 which shows an engine torque versus throttle opening degree characteristic curve with a constant engine revolution speed, the small change in the throttle opening degree causes a change dTE in the engine torque. Thus, since a change in the transmission output torque is the product of the change di in the reduction ratio and the change dTE in the engine torque, this change (di.times.dTE) in the transmission output torque will have a substantially large value.
An object of the present invention is therefore to provide a reduction ratio control system which assures smooth operation and thus increases riding comfort.
A specific object of the present invention is to provide a reduction ratio control system which can prevent unnecessary variations in reduction ratio of a continuously variable transmission during cyclic depression and release operation of an accelerator pedal.